In semiconductor manufacture it often necessary to deposit a thin film over a substrate or over another film on the substrate. One film material that has characteristics suitable for semiconductor manufacture is aluminum nitride (AlN). Among the useful properties of aluminum nitride (AlN) are a high thermal conductivity, a close thermal expansion match to a Si substrate, and good mechanical strength. Table 1 lists the properties of aluminum nitride (AIN):
TABLE 1 ______________________________________ Properties of Aluminum Nitride (AlN) ______________________________________ Dielectric constant 8.8 Resistivity (ohm-cm) 5 .times. 10e13 Bandgap (eV) 6.2 Grain size (um, bulk value) 3-5 Density (g/cm3, bulk value) 3.25 Thermal expansion 2.6 .times. 10e(-6) coefficient (1/K) Flexural strength (MPa) 300 Thermal conductivity (W/cmK) 1.5 Melting point (.degree.C) 2400 Refractive index 2.0 ______________________________________
Such properties allow aluminum nitride (AlN) to be used in semiconductor device packaging and in semiconductor manufacture as an ion implantation mask. In these applications the electronic configuration of both aluminum and nitrogen allow the atoms to assume substitution sites in the silicon crystal lattice structure. Hence they will not introduce or nucleate stacking faults. These properties also allow aluminum nitride (AlN) to be used as a thermally conductive dielectric barrier in various semiconductor devices and to provide a high bandgap window for GaAs solar cells used in semiconductor devices.
Despite these useful characteristics of aluminum nitride (AlN), its relatively high fabrication cost has prevented its wide use in the microelectronic industry. Being a high temperature material, in order to have suitable device characteristics, for some applications, aluminum nitride (AlN) requires formation and fabrication at temperatures so high that many of the materials involved in the process react and adversely affect the electronic properties of the film. Accordingly relatively complex and expensive manufacturing processes are required. U.S. Pat. No. 4,152,182 to Rutz for instance, discloses such a process wherein aluminum nitride (AlN) is synthesized and grown epitaxially on an (Al.sub.2 O.sub.3) substrate. Temperatures in excess of 1900.degree. C. are required. This high temperature process is required to provide a high quality aluminum nitride (AlN) film.
It is also known to deposit an aluminum nitride (AlN) film using chemical vapor deposition (CVD) or sputtering. A (CVD) process, in general will not provide a film as high in quality as an epitaxially grown film but can be used to provide a film with better step coverage. U.S. Pat. No. 4,030,942 to Keenan et al. discloses the use of an aluminum nitride (AlN) film as an ion implantation mask in semiconductor manufacture. A (CVD) process is used to deposit the aluminum nitride (AlN). The disclosed (CVD) process is relatively complicated and requires the use of several process gases including hydrogen, NH.sub.3 and stoichiometric quantities of aluminum chloride. With the use of such a large number of gases, impurities may be introduced into the deposited (AlN) film. These impurities have any adverse affect on the completed semiconductor devices.
In addition to this fundamental problem, both of the cited references are relatively complicated and expensive processes not generally suited to large scale repetitive semiconductor manufacture. In view of the foregoing, there is a need in the art for an improved process for depositing a thin film of high purity aluminum nitride (AlN). Accordingly, it is an object of the present invention to provide a process for depositing aluminum nitride (AlN) using nitrogen plasma sputtering. It is a further object of the present invention to provide an improved process for depositing aluminum nitride (AlN) in a semiconductor manufacturing process having a quality suitable for semiconductor devices. Yet another object of the present invention is to provide a deposition process for aluminum nitride (AlN) that is simple, cost effective and repetitive.
One application where the process of the invention is particularly suited is in the formation of an etchstop layer for semiconductor fabrication. As semiconductor device dimensions continues to shrink, the depth of an implanted area on a Si wafer substrate, such as an active area, becomes increasingly shallower due to the scaling in the device dimensions. This puts a limit on the amount of overetch allowed during a contact etch process through an oxide layer to the substrate, since excessive overetch will consume the Si in an implanted junction, resulting in device degradation. Unfortunately, overetch is often required in a contact etch process in order to (1) making sure that all contact holes are properly etched across the wafer and (2) in cases where two types of contact with two different desired contact depths are present on the same Si wafer. In the later case, in order to open the deeper contact, the shallower contact will be overetched. In order to eliminate Si consumption during any overetch step, an etchstop which consists of a thin layer of a desired material of a slower etch rate than that of the material to be etched (i.e. oxide) can be used. The etchstop layer is usually deposited above the implanted Si substrate (contact junction) and below the oxide layer through which the contact holes will be opened. In a contact etch process, when an etchstop of a much lower etch rate than that of oxide is used, the etch will stop on the etchstop layer during the overetch step, preventing the underlying Si junction from being consumed by the etch process. Therefore, with the help of an etchstop layer between the Si junction and oxide layer, an overetch can be allowed. The process of the invention is especially suited to forming an etchstop in this application.